Neuropeptides are now recognized to be an integral part of chemical neurotransmission in the central nervous system; however, little is known regarding the regulation of the metabolism of this class of transmitters. The proposed research is directed toward defining the dynamics of the intracellular metabolism of the opiate peptide enkephalin in specific projection systems within the rat central nervous system. Several aspects of enkephalin metabolism will be studied using both in vivo and in vitro techniques: (a) the rate of enkephalin utilization in the projection from the caudate-putamen to the globus pallidus and in the mossy fiber system of hippocampal granule cells will be determined in both control and physiologically or pharmacologically stimulated animals. These data will be combined with an analysis of the induced changes in preproenkephalin mRNA and in enkephalin content to arrive at an overall understanding of the regulation of enkephalin biosynthesis. For these studies in vivo pulse labeling of cell bodies with radioisotopic amino acid will be followed by the purification of radiolabeled Met-enkephalin containing peptides from terminal fields using sequential high performance liquid chromatography. (b) Using the same in vivo labeling techniques, the specific activity of the peptides present in the readily releaseable pool will be determined in both control and stimulated animals. (c) The activity of the enzymes responsible for the post-translational liberation of Met-enkephalin from proenkephalin will be studied in secretory granules purified from the striatum and from the hippocampus from both control and stimulated animals. The information obtained from these experiments will be essential in interpreting the potential physiological roles for the enkephalin peptides in these two projection systems and can serve as a basis for examining the metabolism of other CNS neuropeptides.